Sunday, March 16, 2014

Inverted Trophic Pyramids?

Top predator biomass in pristine coral reefs is described to be between 54% and a whopping 85%, meaning that the resulting classical trophic pyramid of biomass is inverted, thus seemingly contradicting the rule that the biomass of prey must be larger than that of the predators because most of the energy gets lost whenever it is moving up through the food chain.

The prevailing explanation for this apparent paradox, is that the lower trophic levels have a much higher turnover rate whereas the top is much slower both in metabolism and reproduction. In fact, if instead of simply taking a snapshot and looking at biomass, one were to to plot the same data over time by depicting a productivity pyramid, one would once again obtain a standard, non-inverted shape.

But is this really the whole story.

Coral reefs are not closed
systems.This is specially true for those remote islands that feature important
influxes both from the open ocean (= plankton but also larger, often migratory transients) and from the
deep, the latter especially at night. This means that the effective
available biomass of prey may in fact be much larger than merely the local residents.

The range of those predators may encompass multiple ecosystems. Research shows that many reef Sharks are resident during the day but disperse during the night, likely in order to forage. Thus, those islands may merely act as anchor points during periods of relative inactivity during the day, whereas feeding may happen both on the reef but also offshore and deeper - once again meaning that for those Sharks, the effective biomass of available prey may well be much larger than what can be deducted by merely observing what lives on those reefs.

And if so, how would the resulting trophic pyramid look like?
Just sayin'! :)

Anyway.

When researching the above, I've stumbled across this inspirational TED talk by Enric Sala.

4 comments:

Dr. D
said...

Don't buy this idea at all...and I'm not alone. All depends on spatial scale of your measurement. You could say this is true, say at Fakrava south pass, but if you expand habitat to entire atoll it would not be true. Sharks aggregate at certain locations which can skew perspective.

The first one is that a lot of big predators have been extirpated and that most of the reefs as we see them today are nothing like they used to be.Having dived for 40 years, I cannot but fully confirm.

The second one is that inverted pyramid thing.Methinks that some very small, very isolated islands and atolls could act as "mega-FADs" and indeed feature such a highly skewed resident fish population structure.

But as you correctly remark, that's likely the exception and it would probably be wrong to generalize and postulate that that applies to how all pristine reef communities were structured before human intervention. Yes there were many more predators - but there were also many more prey fishes and coral.

My recollection is that maybe with the exception of Cocos (but were those thousands of Hammerheads residents - quite certainly not), nothing I've see looked completely predator dominated, and this in then extremely remote and definitely not heavily fished locations.

I do find issue with the methods used to estimate shark abundance as well. I think using dive surveys with large mobile predators are quite problematic, especially in areas where there are few humans and sharks tend to be attracted to divers.